Usually, we need only one exchange that will have all of the cryptocurrency. But with many people, especially for people who have big funds, they see that creating the exchange will give another money, so they raise to build many exchanges in out there. So we can see now, many exchanges available and ready to be used and you are free to choose which exchanges that will suit your need. You don't have to register on more than 5 exchanges if you cannot manage each exchange. You could only have 2 or 3 exchange so you can maintain all of your assets.